1. Field of the Invention
The present invention relates to partial response maximum likelihood (PRML) methods and systems and to the use of tape drives and magnetic tapes to store data.
2. Background Art
In a traditional magnetic storage medium, a read circuit detects flux reversals to determine a data signal. Peak detection is used to interpret the information present in the data signal. As data areal density increases, the peaks get smaller and smaller relative to the background noise, and accordingly, get more and more difficult to detect. A technique used to allow further increases in data areal density that addresses difficulties associated with peak detection is partial response maximum likelihood (PRML).
PRML does not attempt to detect individual peaks in the way that the traditional peak detection techniques do. PRML uses digital signal processing to analyze the analog data signal from the read circuit to determine the most likely pattern of flux reversals. That is, PRML determines the most likely data stream based on the partial response observed in the analog data signal from the read circuit. PRML techniques have been quite successful in allowing the continued increase in areal data density for magnetic storage applications.
The use of tape drives and magnetic tapes to store data has become widespread. Tape drives have many advantages for certain storage applications in that they are able to meet the capacity, performance and reliability needs of these applications at an acceptable cost. PRML approaches have been used in tape drive applications.
Due to the increasing velocity ramp time of modern tape drive systems, it is becoming imperative that the velocity of the transport match the composite transfer rate of the computer host. This requirement leaves the tape system with the task of managing channel transfer function variation over the range of written velocity. Inherently, the transport is aware of the velocity with which it is reading, but is not aware of the velocity with which the data was previously written. Because the channel transfer function varies over the range of written velocity, to appropriately optimize the system, the read detection function must be able to change equalization prior to reading the data. A problem is the fact that it is difficult to appropriately optimize the system because it is difficult to determine the precise way that the equalization needs to be changed.
For the foregoing reasons, there is a need for a method of identifying the velocity with which the data was written that is reasonably immune to write channel transfer function variation.